In the following, again reference is taken to FIGS. 10 to 12.
All illumination designs have to take into account the étendue theorem which states that the étendue or optical extent along an optical system cannot be reduced. For a given surface S the étendue E is defined by the surface S multiplied by the solid angle Ω sustaining the light rays, i.e.E=S·Ω,  (1)according to FIG. 11 for Definition of the étendue E.
In the general system illustrated in FIG. 10, the maximal useful étendue E2 is defined by the surface S2 to illuminate and the solid angle Ω2. For instance, if for the étendue E1 of the source the relation E1>E2 holds, then part of the light is lost.
The solid angle Ω2 is function of the aperture of the optics and is given by the equation
                                          Ω            2                    =                                    2              ⁢                              π                ·                                  (                                      1                    -                                          cos                      ⁢                                                                                          ⁢                                              φ                        2                                                                              )                                                      =                          4              ⁢                              π                ·                                                      sin                    2                                    ⁡                                      (                                                                  φ                        2                                            2                                        )                                                                                      ,                            (        2        )            where φ2 is the half angle of the cone of aperture.
The étendue E1 of the LED array is defined as
                                          E            1                    =                                                    Ω                1                            ·                                                ∑                                      i                    =                    1                                    N                                ⁢                                  S                                      1                    ⁢                    i                                                                        =                          2              ⁢                              π                ·                                  S                  1                                                                    ,                            (        3        )            where S11 is the emission surface of each individual LED, N is the number of LEDs in the array, and 2π is the solid angle of the hemisphere corresponding to the Lambertian emission.100% Collimation Efficiency System
The étendue theorem states that the étendue along an optical system cannot be reduced. Therefore, in order to achieve an optical system with a 100% collimation efficiency, the emission surface S1 of the LED array cannot exceed S1max as is shown by the following relations
                                                        E              1                        ≤                          E              2                                ,                                          ⁢                                    2              ⁢                              π                ·                                  S                  1                                                      ≤                          4              ⁢                              π                ·                                                      sin                    2                                    ⁡                                      (                                                                  φ                        2                                            2                                        )                                                  ·                                  S                  2                                                              ,          and                ⁢                                  ⁢                                            S              1                        ≤                          S                              1                ⁢                max                                              =                      2            ·                                          sin                2                            ⁡                              (                                                      φ                    2                                    2                                )                                      ·                                          S                2                            .                                                          (        4        )            System with Limited Collimation Efficiency
If for the surface of emission the relation S1≧S1max holds, part of the emitted light will not reach the surface S2 within the aperture φ2, and will therefore be lost.
The problem is analysed by looking at what is the emitted cone or half-angle φ1 at the surface S1 which is within the aperture of the optics or half-angle φ2 at the surface S1.
From the étendue theorem it follows that
                    S        1            ·      4        ⁢          π      ·                        sin          2                ⁡                  (                                    φ              1                        2                    )                      =                    S        2            ·      4        ⁢          π      ·                        sin          2                ⁡                  (                                    φ              2                        2                    )                    is fulfilled. Therefore,
                              φ          1                =                  2          ·                                    sin                              -                1                                      [                                                                                S                    2                                                        S                    1                                                              ·                              sin                ⁡                                  (                                                            φ                      2                                        2                                    )                                                      ]                                              (        5        )            is also fulfilled.
The coupling efficiency ηc is defined as the ratio of the emitted energy W1 within the cone defined by φ1, and the total energy W emitted by the source or surface S1, i.e.:
                              η          c                =                                            W              1                        W                    .                                    (        6        )            
In the case of a Lambertian light source with an emission angle γ=π/2, the coupling efficiency becomes
                              η          c                =                                                            ∫                                  -                                      φ                    1                                                                    +                                      φ                    1                                                              ⁢                                                ∫                                      -                                          φ                      1                                                                            +                                          φ                      1                                                                      ⁢                                                      cos                    ⁡                                          (                      α                      )                                                        ⁢                                                            cos                      ⁡                                              (                        β                        )                                                              ·                                          ⅆ                      α                                        ·                                          ⅆ                      β                                                                                                                          ∫                                                      -                    π                                    /                  2                                                                      +                    π                                    /                  2                                            ⁢                                                ∫                                                            -                      π                                        /                    2                                                                              +                      π                                        /                    2                                                  ⁢                                                      cos                    ⁡                                          (                      α                      )                                                        ⁢                                                            cos                      ⁡                                              (                        β                        )                                                              ·                                          ⅆ                      α                                        ·                                          ⅆ                      β                                                                                                    =                                    sin              2                        ⁢                                          φ                1                            .                                                          (        7        )            
The luminous flux W2 reaching the surface S2 within the aperture φ2 is proportional to the emission surface S1 of the source and to the collimation efficiency ηc,W2∝ηc·S1.  (8)
Three cases can be distinguished:    1) S1≦S1max and ηc=1, all the light emitted by the source can be used: W2∝S1,    2) S2≦S1>S1max and ηc<1, part of the light is lost, but as the surface of emission S1 increases, W2 increases, and    3) S2>S1, the surface of emission S1 increases, but W2 does not increase.Proposed Solution and Features
Different solutions based on reflectors and/or refractive lenses have been proposed for the collimation of LEDs. The drawback of these known approaches is that it is difficult to collect 100% of the light in the desired direction. Moreover the optics surrounding the LED is cumbersome, artificially increasing the étendue of the source. In addition, further optics is needed to make the illumination uniform, e.g. fly-eye lenses or an integration rod.
According to the present invention an approach based on—in particular pyramidal shaped—integration rods is proposed. This approach fulfils the three needed functions of                collecting the light emitted by the LED array,        collimating within the aperture of the optics, and        homogenising the illumination.        
The working principle of a pyramidal integration rod or PIR is illustrated in FIG. 5. The PIR has an entry surface S′, an exit surface S″, and length L. The PIR can be an empty tube whose internal faces are mirrors, or a plain transparent material—e.g. mineral glass, plastic or the like—of index n. For a plain PIR, the rays are reflected on the surface by total internal reflection or TIR. As is illustrated for two rays in FIG. 5, the angle with respect to the PIR surfaces normal is smaller at the exit of the pipe than at its entrance. Given the étendue theorem, the collimation is defined as
                              Ω          ″                =                                            S              ′                                      S              ″                                ·                                    Ω              ′                        .                                              (        9        )            where Ω′ is the solid angle of the ray before the PIR, and Ω″ is the solid angle of the ray after the PIR. The relations S″>S′ and Ω″<Ω′ are fulfilled.
As for a normal integration rod, the rays are mixed within the rod. Two condition have to be fulfilled in order to get an uniform distribution at the PIR exit surface:
1. The PIR cross-section has either to be square, rectangular, (in particular equilateral) triangular, or hexagonal.
2. The PIR has to be long enough to allow enough reflections against the PIR surface.
The theoretical collimation efficiency ηc is achieved for L≧Lc. Above the length Lc the collimation efficiency is constant. Lc is determined experimentally or by ray-tracing simulation, in a case by case basis.
In problem described above, the PIR entry surface S′ has to coincide with the LED emission surface S1, and the PIR exit surface S″ has to coincide with the surface S2 to illuminate. As the LED array is constituted by a set small emission surfaces S11, a micro PIR can be placed in front of each LED. The light is then collected by a bigger PIR or integration rod. The three systems illustrated in FIG. 6 are all equivalent, given the length of the PIR is long enough to complete the collimation and the homogenisation.
Main advantageous features of the present invention are the usage of a single optical element is used for light collection, light collimation, and light homogenisation. By using a single component from the light source (LED array) to the illuminated plane, the proposed approach minimizes the optical loses, lowers the manufacturing costs, and simplifies the device assembly.                These and further aspects will also be elucidated in the following:        
The invention proposes inter alia an illumination scheme based on a colour multiplexer and light pipes. It allows the uniform illumination of a surface (e.g. a m-display) by the combining the light of different colour light sources (e.g. red, green, and blue LEDs). The invention consists in the combination and particular assembly of the different optical components allowing an extremely compact embodiment.
Colour combination can either be achieved by using a suite of dichroic filters or by using a colour cube. The combination of dichroic filters (coated glass plates) allows the combination of different colour beams into a single polychromatic beam. The coatings reflect one single colour (e.g. green or red) and transmit all the others. Note that the suite of dichroic filters can also be applied on prisms.
The colour cube is formed by the assembly of four prisms. The prism surfaces forming the cube diagonals are coated in such a way to reflect one colour (e.g. red or blue) and to transmit all other colours. In this way, three different colour beam can be recombined in a single polychromatic output beam.
It should be noted that these colour combination schemes do not fulfil any function in order to homogenize the illumination produced by the output beam.
A illumination engine has been developed using a pyramidal light pipe. The light pipe fulfils the following functions: collection of the light coming from the light source (or array of light sources), collimation of the light coming from the source(s), and homogenisation of the illumination. It should be noted, that an array of different colour sources can be used. In this way a polychromatic, uniform, and collimated illumination is achieved.
However, the limited surface of the light pipe input surface determines the maximal number of source elements, and therefore limits the brightness of the source
A problem consists in finding a configuration which has the same functionality of the illumination set-up presented with light collection, light collimation, homogenisation of illumination, but with a increased surface available for the sources (allowing an increased brightness.
A key parameter (requirement) is the compactness of the embodiment (e.g. for illumination of m-displays).
A basic idea in order to increase the brightness of the colour illumination device is to combine the properties of the colour combination schemes described, in particular with the 2nd proposal below, and the light pipe illuminator described, in particular in the 3rd proposal below. One goal is to have a bigger surface for coupling the light sources (e.g. LEDs) and to achieve a uniform and collimated illumination with a pyramidal light pipe.
1st Proposal:
The first proposed set-up, consists of a colour cube for collecting and multiplexing the light of the single colour light sources, and a pyramid light pipe for the homogenisation and collimation of the illumination, see FIG. 13.
Note that an air gap is required between the light sources and the colour cube as well as between the colour cube and the light pipe. The air gap reflects by TIR (total internal reflection) the rays which would otherwise escape the cube. These rays would either be absorbed or propagate in an undesired direction, producing optical losses. Note that some rays are practically unaffected by the air gap
2nd Proposal:
The second proposal uses a sequence of dichroic filters to combine the different colour sources, see FIG. 14.
It should be noted that the dichroic filters are in glass cubes, called hereafter dichroic cubes. The light sources, the dichroic cubes, and the light pipe are separated by an air gap. As for the 1st proposal, the air gap allows to guide ray by TIR and minimize optical losses. The use of dichroic filter on glass plates would also let rays escape in undesired directions producing optical losses.
The 2nd proposal is less compact than the 1st proposal, but it is simpler technologically speaking. It is therefore cheaper to realize, in particular for very compact dimensions.
3rd Proposal:
For compact configuration, the first two proposals may be difficult to manufacture. For example, in the case of the illumination of a 0.7″ LDC m-display, the cube edge has a typical size between 2.0 mm and 4.0 mm.
A way to relax the cube dimension constraint is shown in FIGS. 15 and 16. The two configurations are functionally equivalent. However the positioning of the cube in an intermediate position is compacter
It should be noted that the cube is surrounded by air gaps in order to minimize optical loses, as already explained. However, when the source is directly in front of the light pipe, there is no need to include an air gap between the source and the light pipe. Actually when using surface mounted LEDs, the light extraction efficiency of the source is increased when the pipe is in contact with the LED surface. This results in significant increase of the overall optical efficiency.
The semiconductor active surface emits rays in random directions. Due to the high refractive index of the semiconductor when compared to the refractive index of the epoxy layer, part of the rays are trapped by TIR. The same happens for part of the rays between the epoxy and the air gap. However when the epoxy is in contact with the light pipe, all the rays leaving the epoxy are coupled into the light pipe, as the refractive index of the epoxy and the refractive index of the pipe can be (need to be) chosen close to each other (close to index matching condition).
Reminder: total internal reflection condition
For a ray in a high refractive index medium reaching the interface with a lower refractive index medium, if the angle of incidence is larger than the critical angle, then the ray is totally internal reflected. Otherwise the ray is refracted and propagates in the low refractive index medium, see FIG. 26.
Prism Coating:
When using colour or dichroic cubes for the proposed illumination configurations, the full dichroic surface is used. In other words, the full surface should be coated by the dichroic filter. In practice however, the coating does not stick to the surface close to the edges (see FIG. 12). When the prism size is small, the uncoated margin represents a significant proportion of the surface. This results in optical loses as the colours rays are not correctly redirected when falling on the uncoated margins.
A way to turn around the uncoated margin problem consists of coating a thin glass plate whose dimensions are bigger than the prism cross-section. The plate is then glued between two prisms. The uncoated margin being out of the prisms cross-section, the efficiency of the dichroic filter is optimised.
The proposal inter alia allows to combine sources of different colours and to uniformly illuminate a surface in a very in compact embodiment; this with minimal optical losses. Our proposal also show how to relax the dimension requirements on the colour cube.                These and further aspects will also be elucidated in the following:        
The present invention inter alia also relates to a light extraction mechanism for LED illuminators.
The optical efficiency of a LED based illumination device depends on the design and assembly process between the light source and the collimation optics. We present an original optimised design as well as an assembly/manufacturing technique suitable for low cost mass-production.
The outcoupling or extraction efficiency of LEDs is generally done by the LED manufacturer by applying a micro-structure on the chip surface or by giving a special shape to the encapsulating material (e.g. LED with epoxy lens).
The invention inter alia proposes an illumination architecture based on light pipes. It is further proposed a system for which two optical elements are coupled with a index matching fluid.
Some aspects of the present invention consist of:                improved light pipe efficiency by combining total internal reflection (plain pipe) and mirror reflection (hollow pipe), and        conception of a assembly scheme for efficient lighting and low cost mass-production.        
The invention inter alia intends to provide solutions for the following main problems:    1) An assembly technique is proposed for low cost mass-production.    2) The combination of a plain and hollow pipes optimises the device illumination efficiency.
As explained above, the combination of a LED array and a pyramidal light pipe constitutes an efficient and compact illumination system which can be, for example, used for the illumination of micro-displays used in projection application.
The light pipe can either be a plain light pipe for which the rays are guided by total internal reflection (TIR), or an hollow pipe whose internal faces are mirror coated (metallic reflection). The advantage of plain pipes is that the redirection of ray by TIR is loss less. The advantage of hollow pipes is that they can be made shorter, as the rays propagating in air have larger angles and meet the light pipe faces after a shorter distance. On the other hand, the mirror reflection of hollow pipe produces optical losses (typical mirror reflectivity varies between 92% to 98% depending on angle of incidence and mirror material).
Key parameters of such an illumination engine are:    1) the illumination efficiency    2) the uniformity of illumination, and    3) the compactness of the device.
The illumination efficiency η is defined as
  η  =                    ϕ        opt                    ϕ        LED              .  where φLED is the flux emitted by an LED and φopt is the flux provided by the illumination engine within the limited aperture (solid angle) of the optics.
The state-of-the art of the LED light pipe illuminator suffers of following defaults    A. For plain light pipes: non optimal optical efficiency due to ray escaping the light pipe laterally, see FIG. 29A.    B. For plain light pipes: non optimal optical efficiency due to non perfect index matching between the LED source and the light pipe, see FIG. 30A.    C. For hollow pipes: non optimal efficiency due to poor LED extraction efficiency, see FIG. 31A.
As explained hereafter, the invention inter alia consists in avoiding these three shortcomings, as well as in proposing an efficient assembly solution well suited for mass production.
ad A: Optimisation of Light Guiding
Plain light pipes suffer some losses as some ray reach the pipe faces out of the TIR condition. Note that this effect only happens for rays with large propagation angles at the beginning of the pipe, where the rays have still not been deflected.
It is proposed to overcome the loss of optical efficiency produced by the rays escaping laterally by coating the first section of the light pipe with a mirror. Alternatively, the plain light pipe can be plugged into a small hollow pipe. This way, the rays are guided by metallic reflection in the first section of the light pipe (with some minimal reflection losses), but are guided by TIR for the rest of the light pipe (without losses), see FIG. 29B.
ad B: Optimisation of LED-to-Pipe Coupling
The light extraction efficiency of LEDs is limited by the rays which are trapped by TIR. The TIR is the consequence of the difference of index between the LED protective layer (e.g. epoxy or silicone) and the air. This can be avoided when there is an refractive index matching between the LED and the light pipe. A good index matching is achieved when there is no air gap between the LED protective layer (and the light pipe (e.g. PMMA). In practice, this is difficult to achieve due to the imperfect surface flatness of the LED protective layer. Typical indices vary with wavelength and gave values for silicone of about 1.46, for epoxy of about 1.53, and for PMMA of about 1.49.
The way to avoid any the air gap between the LED and the light pipe is to insert a fluid (liquid, gel, and/or glue) whose index of refraction corresponds to those of the pipe and the LED protection layer. Note that due to capillarity forces the fluid sticks between the two parts, see FIG. 30B.
ad C: Optimisation of Hollow Pipes Efficiency
Due to their nature, hollow pipes cannot improve LED extraction efficiency by index matching, as LED-to-pipe coupling is not possible. To avoid this problem a small plain pipe section whose role is to increase LED extraction efficiency is inserted to make a first ray redirection. This way the advantage of hollow pipes (shorter that plain pipe) and plain pipes (high extraction efficiency) is combined, see FIG. 31B.
D: Optimisation of Assembly Process
The LED-to-pipe assembly process has to fulfil the following conditions:    1. Fix together the optical components (LED and light pipe)    2. Preserve the optical properties of the device (high optical efficiency)    3. Be as simple as possible in order to reduce costs of the mounting process for mass-production
The invention is based on a configuration which, as discussed, has a high optical efficiency. In order to keep costs as low as possible, it is proposed to use a glue as index matching material and to use the specially shaped hollow pipe in order to fix the different components laterally.
Possible steps of the assembly process are:    1) Position the specially shaped light pipe on top/around of the LED    2) Deposit a droplet of index matching glue    3) Press the plain light pipe against the LED    4) Fine tune the horizontal alignment of the light pipes and the LED    5) Cure of the glue. Depending on the glue, the curing is either done thermally, by infrared illumination, or by ultraviolet illumination.
In order to maximize the light extraction efficiency, it is important that the volume of the glue droplet is big enough to fill the air gap volume between the plain pipe and the LED. The air gap volume between the LED and the pipe is hard to predict with precision as it depends on the LED manufacturing process (deposition process of the epoxy). By precaution, the volume of the glue is chosen with some margin. As illustrated in FIGS. 32A and 32B, the glue in excess finds its place is the space available between the lateral faces of the LED and the hollow pipe basis.
The invention solves the following problems of the state-of-the art:                For plain light pipes: non optimal optical efficiency due to ray escaping the light pipe laterally.        For plain light pipes: non optimal optical efficiency due to non perfect index matching between the LED source and the light pipe.        For hollow pipes: non optimal efficiency due to poor LED extraction efficiency.        
Moreover, the invention proposes a cheap and efficient assembly process optimised for mass-production.
                The following description is directed to preferred embodiments of the present invention, in particular with respect to said first solution by taking reference to FIGS. 1 to 12.        
FIG. 1 demonstrates by means of a schematical and cross-sectional side view a first preferred embodiment of the inventive illumination arrangement 1.
The embodiment of FIG. 1 consists of a light source device 10, which is built-up by a solid state light source device 30. The solid state light source device 30 of the embodiment of FIG. 1 consists of an array 33 of light emitting diodes 31. Said array 33 is formed to have a light emitting area or light emitting aperture 30E from which primary illumination light L1 is emitted to reach an incidence aperture 50I of a light collecting, integrating and redirecting device 20 which may consist as in the example of FIG. 1 of a light integrating device 50 and of a light valve device 40, the former of which is in this case formed as an integration or integrator rod 50 of a solid bulk material, for instance glass, plastic, or the like.
Rays of primary illumination light L1 entering said integrator rod 50 via said light incidence aperture 50I are first of all refracted according to the Snell's law of refraction and according to a refractive index of the material of the integrator rod 50 being larger than the refraction index of the gap material of the gap G between said integrator rod 50 and the light source device 10. During the passage of the primary illumination light rays L1 within the material of the integrator rod 50 said rays of light are reflected at the side walls or faces 50s of the integrator rod 50. Finally, after a plurality or multiplicity of reflections at the side walls 50s each of said received and multiply reflected rays of light of the primary illumination light L1 exits from the integrator rod 50 via output aperture 50E and then enters the light valve 40 being situated in direct proximity to the light output aperture 50E.
After exiting said integrator rod 50 via output aperture 50E, the light distribution in the second gap G′ between the integrator rod 50 and the light valve 40 is more uniform than the light distribution at the first gap G between the light source device and the integrator rod 50.
After receiving the redirected primary illumination light RL1 the respective rays of light are allowed to pass through the light valve 40 in a controllable manner and they leave the light valve 40 as secondary illumination light L2 to enter certain projection optics 70, shown in FIG. 2, and then entering a display screen 80.
The gap G between the light source device 10 and the integrator rod 50 which is shown in FIGS. 1 and 2 is of particular importance as even in the case of an array of light emitting diodes each of said diodes 31 only has a minor directive emission capability because the light distribution or energy distribution of emitting light waves obeys a Lambertian distribution as shown in FIG. 3. FIG. 3 demonstrates this Lambertian distribution as a graph demonstrating the relative energy of emitted light for a light emitting diode 31 as a function of the emission angle. From the distribution of FIG. 3 it can be derived, that it is necessary to keep the gap width of the gap G between the light source device 10 and the integrator rod 50 as narrow as possible to increase the integral or the amount of primary illumination light L1 entering the area of incidence or incidence aperture 50I.
As can be seen from FIG. 2, the cones of acceptance of the integrator rod 50 and the displaying optics 70 may be different. Therefore, it could be necessary to adapt said cones of acceptance. This can be done alternatively by employing fly-eye-optics as shown in FIG. 4A or more preferably by using an integrator rod 50, having a pyramidal cross-section as shown in FIG. 4B.
FIGS. 5 and 7 to 9 demonstrate different possibilities of combining solid state light source devices 30 of different colours to obtain a multi-colour illumination arrangement for a multi-colour projection system.
In FIG. 5 three different coloured solid state light source devices 30 having e.g. light emitting diode arrays 33 are given, each of said solid state light source devices 30 and therefore each of said light emitting diode arrays 33 being associated with an integrator rod 50 interposed between said solid state light source device 30 and a light valve arrangement 40, so that for each of said light source devices 30 of different colours an arrangement similar to that shown in FIG. 1 is given.
To combine the three different colours of said three different solid light source devices 30 a light mixing device 55 or colour cube 55 common for each of said three arrangements is given being capable of receiving the respective secondary illumination light L2, to mix them up, and to allow them to pass over to the projection optics 70.
FIG. 6A to 6C show different embodiments of the light collecting, integrating and redirecting unit or device 20 in the form of different integrator rod arrangements each of which being adapted for an array 33 of LEDs 31 or 31-1 to 31-4 as a light source device 10 and each of which being optically coupled to a light valve device 40.
In FIG. 6A the light collecting, integrating and redirecting unit or device 20 is formed as a plurality of more or less similar or identical separated and parallely arranged single pyramidal integrator rods 50-1 to 50-4 each of which being uniquely assigned and coupled with its respective light entrance section 50I to a given LED 31-1 to 31-4, respectively. The light entrances 50I are in each case smaller than the respective light output sections 50O which are optically coupled to the light entrance section 40I of a common light valve device 40.
In FIG. 6B the more or less similar or identical separated and parallely arranged single pyramidal integrator rods 50-1 to 50-4 are optically coupled instead to the light entrance section 50I′ of a common and integrator rod 50 the light exit 50O of which being optically coupled to the light entrance section 40I of a common light valve device 40.
The common integrator rod 50 of the embodiment of FIG. 6B has a uniform cross section, whereas the cross section of common integrator rod 50 of the embodiment of FIG. 6C is non-uniform and the respective integrator rod 50 is formed pyramidal.
FIGS. 7A and 7B demonstrate two different arrangements for realizing multiple colour illumination arrangements for multiple colour projection systems which differ from the embodiment of FIG. 5.
In the embodiment of FIG. 7A a solid state light source device 30 is employed as said light source device 10 which has a LED-array 33, the members of which, i.e. the distinct light emitting diodes 31, possessing different spectral emission ranges, i.e. different colours. After the passage of the primary illumination light L1 through the integrator rod 50 at the gap G′ between the light valve 40 and the integrator rod 50, the uniform light distribution and the uniform colour distribution after passing the light valve 40 is then directed to the projection optics 70.
In the case of the embodiment of FIG. 7B three different coloured solid state light source devices 30, each of which being built-up by an array 33 of light emitting diodes 31 have distinct spectral ranges or colours with respect to each other. The primary illumination light L1 of each of said single solid state light source devices 30 is directed to the light mixing device 55 which after mixing directs the output light to the integrator rod 50 to obtain a secondary illumination light L2 at the gap G′ between the light valve 40 and the integrator rod 50 having a uniform illumination and colour distribution.
FIGS. 8A and 8B demonstrate further examples of multiple colour illumination devices. In these cases illumination light pipes 50 are used for redirecting and making uniform received amounts of primary illumination light L1. In contrast to the embodiments discussed above, where the incidence aperture 50I at which primary illumination light L1 enters the distinct integrator rod 50 and the output aperture 50E are disposed in parallel to each other, the incidence apertures 50I and output apertures 50E of the embodiments of FIGS. 8A and 8B are perpendicular to each other. Therefore, primary illumination light L1 emitted by solid state light source devices 30 of the embodiments of FIGS. 8A and 8B enters the associated illumination light pipes 50 at their base faces, whereas the redirected primary illumination light RL1 exits from said illumination light pipes 50 at side faces thereof.
A difference between the embodiments of FIGS. 8A and 8B is that for obtaining a multi-colour arrangement in FIG. 8A a plurality of single coloured solid state light source devices 30 or LED-arrays 33 is necessary, whereas in the embodiment of FIG. 8B multiple coloured solid state light source devices 30 or LED-arrays 33 are provided.
Of course, in the embodiment of FIG. 8A according to the multiplicity of single-coloured solid state light source devices 30 again a light mixing device 55 is necessary.
The embodiment of FIG. 9 demonstrates an application of the embodiment of FIG. 7B, having intermediate optics 81, 82 for adapting the cones of acceptance between the integrator rod 50 and the light valve 40. The intermediate optics 81, 82 consists of a lens arrangement 81 and a polarization beam splitter 82 which in combination with each other transforms or maps the cone of acceptance of the integrator rod 50, i.e. the geometry of the redirected primary illumination light RL1, to the cone of acceptance of the light valve 40, which is in the embodiment of FIG. 9 a reflective light valve 40 which allows the passage of secondary illumination light L2 to the projection optics 70 upon reflection at the interface of light valve 40.                In the following description, some general remarks about dichroic filters and color cubes and further remarks with respect to the present invention, in particular with respect to said second and third solutions will be given by taking reference to FIGS. 13 to 28.        
Color combination can either be achieved by using a combination of dichroic filters or by using a color cube.
FIG. 22 shows a combination of a first dichroic filter 10′ and a second dichroic filter 11′. The two dichroic filters 10′, 11′ are realized as glass plates being coated by respective transmissive/reflective films 16′, 17′. Each dichroic filter reflects one single color (for example red or green) and transmits all other colors. In this example, the first dichroic filter 10′ transmits a first color beam 12′, having for example blue color and reflects a second color beam 13′, of for example red color. The second dichroic filter 11′ transmits the first color beam 12′ and the reflected second color beam 13′ and reflects a third color beam 14′, of for example green color. Thus, a combined color beam 15′ is generated.
As can be taken from FIGS. 23 and 24, combinations of dichroic filters can also be realized as combinations of coated prisms of for example glass. The combination of dichroic filters shown in FIG. 23 comprises a first to third prism 20′ to 22′. Between the first prism 20′ and the second prism 21′ a first reflective/transmissive film 23′ is provided. Further, between the second prism 21′ and the third 22′ a second reflective/transmissive film 24′ is provided. The first transmissive/reflective film 23′ reflects the first color beam 12′ of, for example, blue color. The second transmissive/reflective film 24′ reflects the third color beam 14′ of, for example, green color and transmits the second color beam 13′ of, for example, red color. The reflected third color beam 14′ and the transmitted second color beam 13′ pass the first transmissive/reflective filter 23′. As a consequence, a combined color beam 15′ is generated.
FIG. 24 shows another example of a combination of dichroic filters being realized by coated prisms. In this combination, a first to third prism 25′ to 27′ are provided. Between the first prism 25′ and the second prism 26 a first transmissive/reflective film 28′ is provided. Between the second prism 26′ and the third prism 27′ a second transmissive/reflective film 29′ is provided. Analogous to the embodiment shown in FIG. 23, the properties of the first and second transmissive/reflective films are chosen in a way that the color beams 12′ to 14′ are combined into a combined color beam 15′.
FIG. 25 shows an example of a color cube. A color cube 30′ comprises a first to fourth prism 31′ to 34′. The prism surfaces forming the cube diagonals are coated in such a way that one color (for example, red or blue) is reflected, and all other colors are transmitted. Thus, a first transmissive/reflective film 35′ and a second transmissive/reflective film 36′ are provided. As a consequence, the color beams 12′ to 14′ are combined into a single combined color beam 15.
All examples of light mixing devices given above show possibilities to recombine different color beams into one single polychromatic output beam. It should be noted that these color combination schemes do not fulfill any function in order to homogenize the illumination produced by the output beam.
Making reference to FIG. 13, a first preferred embodiment of an illumination arrangement according to the present invention will be described. An illumination arrangement 40′ comprises a first to third light source 41′ to 43′, a color cube 44′ showing a first transmissive/reflective film 45′ and a second transmissive/reflective film 46′, a pyramidal light pipe 47′, and a target surface 48′ to be illuminated.
The function of the color cube 44′ is to collect and multiplex different color beams generated by the light sources 41′ to 43′. For example, the first light source 41′ produces a blue color beam being reflected by the first film 45′, whereas the third light source 43′ generates a second color beam of red color being reflected by the second film 46′. The color beam of green color being generated by the second light source 42′ passes both the first and the second film 45′, 46′. Thus, a combined single output beam passes through an output surface 49′ of the color cube 44′ and is coupled into the pyramidal light pipe 47′ through an input surface 50′ of the pyramidal light pipe 47′. In this example, the lengths and the widths of output surfaces of the light sources 41′ to 43′ are equal to the lengths and the widths of respective input surfaces of the color cube 44′ (for example the dimensions of an output surface 51′ of the first light source 41′ is equal to that of an input surface 52′ of the color cube 44′). However, the dimensions of the output surfaces of the light sources 41′ to 43′ may also be smaller than those of respective input surfaces of the color cube 44′.
Compared to the embodiment shown in FIG. 21, the brightness of the color illumination of the surface 48′ is remarkably higher since an overall output surface of the light sources 41′ to 43′ which emits light is three times higher than that in FIG. 21. The pyramidal light pipe is responsible for homogenization and collimation of the illumination.
Making reference to FIG. 14, a second embodiment of an illumination arrangement according to the present invention will be described. In this embodiment, the color cube 44′ of FIG. 13 is replaced by a combination of dichroic filters. An illumination arrangement 60′ comprises a first to third light source 61′ to 63′, a first dichroic filter 64′, a second dichroic filter 65′, a pyramidal light pipe 66′, and a surface 67′ to be illuminated. The first and the second dichroic filter 64′, 65′ are realized as glass cubes having a first and a second transmissive/reflective film 68′ and 69′. The first film 68′ combines light beams being generated from the second and third light source 62′, 63′ into a combined light beam which enters the second dichroic filter 65′ via an input surface 70′. The second dichroic filter 65′ combines said combined light beam and a light beam being emitted from the first light source 61′ by means of the second film 69′ to generate a second combined light beam which enters the pyramidal light pipe 66′. The embodiment of FIG. 14 is less compact than that of FIG. 13. However, the illumination arrangement of FIG. 14 is easier to manufacture, in particular for very compact dimensions.
Preferably, in the embodiments of FIGS. 13 and 14, air gaps G are provided between the light sources 41′ to 43′, 61′ to 63′, and the color cube 44′/dichroic filters 64′, 65′. Preferably, air gaps are also provided between the color cube 44′ and the pyramidal light pipe 47′ as well as between the second dichroic filter 65′ and the pyramidal light pipe 66′ and between the first and the second dichroic filter 64′, 65′. The reason for this is explained in FIGS. 17 and 18.
As can be taken from FIG. 17, a first to fourth light ray 53′ to 56′ is generated by the light sources 41′ to 43′. Due to the air gaps G, all light rays 53′ to 56′ are reflected by total internal reflection (TIR) by the air gaps G and are thus coupled into the pyramidal light pipe 47′. Without such an air gap, only one of those light rays 53′ to 56′ would have been coupled into the pyramidal light pipe 47, namely light ray 54′, as can be taken from FIG. 18. All other light rays are either absorbed by the light sources 41′ to 43′ or lost. That is, without air gaps G, the rays would either be absorbed or propagate in undesired directions producing optical losses. Some of the light rays (ray 54′ in FIG. 18) are practically unaffected by the air gaps G. Optical losses is also the reason to employ glass cubes in FIG. 14 and not only glass plates 10′, 11′ as shown in FIG. 22. Such coated glass plates would let light rays escape in undesired directions producing optical losses.
Making reference to FIG. 15, a third embodiment of an illumination arrangement according to the present invention will be described. An illumination arrangement 80′ comprises a first to third light source 81′ to 83′, a color cube 84′ showing a first transmissive/reflective film 89′ and a second transmissive/reflective film 90′, a first to third pyramidal light pipe 85′ to 87′ and a target surface 88′ to be illuminated. Preferably, in this embodiment, the light pipes 85′ to 87′ and the respective light sources 81′ to 83′ are in direct mechanical contact with each other. This embodiment is easy to manufacture since the dimensions of the color cube 84′ are relatively large. To give an example: The cube edge has a typical size of 2.0 mm to 4.00 mm in the configurations of FIGS. 13 and 14, whereas the color cube has the dimensions of the μ-display (e.g. 0.7″ diagonal) in the configuration of FIG. 3. However, the present invention is not restricted to these dimensions. Air gaps G are provided between the light pipes 85′ to 87′ and the color cube 84′. Light beams being emitted by the light sources 81′ to 83′ is transported by the light pipes 85′ to 87′ to the color cube 84′, respectively, which mixes the light beams by means of the films 89′, 90′ to generate a combined light beam which illuminates the target surface 88′.
Making reference to FIG. 16, a fourth preferred embodiment of an illumination arrangement according to the present invention will be described. An illumination arrangement 100′ comprises a first to third light source 101′ to 103′, a color cube 104′ showing a first transmissive/reflective film 105′ and a second transmissive/reflective film 106′, a pyramidal light pipe 107′, a target surface 108′ to be illuminated, and a first to third additional pyramidal light pipe 109′ to 111′. This embodiment differs from that shown in FIG. 13 only by the additional pyramidal light pipes 109′ to 111′. Preferably, in this embodiment, the additional pyramidal light pipes 109′ to 111′ and the respective light sources 101′ to 103′ are in direct mechanical contact with each other.
The embodiment shown in FIG. 16 is more compact than that of FIG. 15. It is not as compact as that of FIG. 13, but easier to manufacture. Air gaps G are provided between the light pipes 109′ to 111′ and the color cube 104′ and between the color cube 104′ and the pyramidal light pipe 107′.
The color cube 44′ and the dichroic filters 64′, 65′ are surrounded by air gaps G in order to minimize optical losses, as already explained. However, when the source is directly in front of the light pipe, as shown in FIGS. 15 and 16, there is no need to include an air gap between the source and the light pipe. This will be explained while making reference to FIGS. 19 and 20.
Actually, when using surface mounted LEDs, the light extraction efficiency of the light source is increased when the light pipe is in contact with the LED surface (see FIG. 20). This results in an significant increase of the overall optical efficiency.
In FIGS. 19 and 20, a semiconductor active surface 121′ of a semiconductor 120′ is shown which emits light rays in random directions. Due to the high refractive index n1 of the semiconductor 120′ compared to the refractive index n2 of an epoxy layer 122′, part of the rays are trapped by total internal reflection (for example ray denoted by reference symbol 123′). In the configuration of FIG. 19, the same happens for parts of the rays between the epoxy layer 122′ and an air gap G (see totally reflected ray denoted by reference symbol 124′). However, in the configuration shown in FIG. 20, where the epoxy layer 122′ is in contact with a light pipe 126′, all the rays leaving the epoxy layer 122′ are coupled into the light pipe 126′, as the refractive index of the epoxy layer 122′, n2 and the refractive index of the light pipe 126′, n3 preferably are chosen close to each other (close to index matching condition).
The principle of total internal reflection is shown in FIG. 26. For a ray in a high refractive index medium n1 reaching an interface 130′ with a lower refractive index medium n2, if the angle of incidence α is bigger than the critical angle αc, then the ray is totally internal reflected (TIR). Otherwise, the ray is refracted and propagates in the low refractive index medium. The critical angle is αc=sin−1(n2/n1).
When using color or dichroic cubes for the illumination configurations proposed above, the full dichroic surface of the prisms/glass plates etc. is “used” (illuminated) by the light beams. In other words, the full surface should be coated by the corresponding dichroic films. In practice, however, it may be difficult to coat the surface close to the edges for several reasons (for example because the edges are already covered with undesired material).
This situation is shown in FIG. 27: A prism surface 140′ is coated by a transmissive/reflective (dichroic) film 141′ as described above. Usually, an uncoated margin 142′ is left because of restrictions in the manufacturing process. When the prism size is small, the uncoated margin represents a significant proportion of the prism surface 140′. This results in optical losses as the color rays are not correctly redirected when falling on the uncoated margin 142′.
A way to turn around the uncoated margin problem is shown in FIG. 28. A thin glass plate 145′ whose surface dimensions are bigger than the surface dimensions of a prism cross-section of two prisms 146′, 147′ is coated with said transmissive/reflective film 141′ and then provided (glued) between the two prisms 146′, 147′ to get a dichroic filter on a glass prism basis. The uncoated margin 142′ does then not lie within the prisms cross-section, thus the efficiency of the dichroic filter is optimized.                The following description is directed to preferred embodiments of the present invention, in particular with respect to said fourth solution by taking reference to FIGS. 29A to 32B.        
FIGS. 29A and 29B demonstrate by means of cross-sectional side views embodiments of the present invention with and without the inventive light collecting and/or guiding improving arrangement 50A, respectively.
For collecting and integrating primary illumination light L1 from a light source device 10 a light integrating device 50 in the form of a pyramidal light pipe or integration rod 50 is provided, the latter having the light incidence aperture 50I and side walls 50s. The integration rod 50 uses the principle of total internal reflections or TIR as is demonstrated by the collected ray C which exits the light source device 10 and its housing 30c through a light exit aperture 30E so as to enter the light integrating device 50 or integration rod 50 through its light incidence aperture 50I. This happens under an angle which is sufficient so as to satisfy the TIR conditions for said collected ray C.
However ray R of FIG. 29A exits the light source device 10 through its light exit aperture 30E under an angle such that the TIR conditions cannot be fulfilled by said ray R. Therefore said ray R is not reflected back to the internal at the side wall 50s of said light integration device 50 but escapes from the light integration device 50. Therefore according to the situation of FIG. 29A light may be lost thereby decreasing the efficiency of the arrangement of FIG. 29A.
According to a further aspect of the present invention the arrangement of FIG. 29A is modified by providing at the periphery of the side walls 50s of the light integration device 50 in the neighborhood of said light incidence aperture 50I reflecting means 50m as said light collecting and/or guiding improving arrangement 50A or as a part thereof. According to this particular measure said ray R which is lost in the arrangement of FIG. 29A is reflected back by said reflecting means 50m of said light collecting and/or guiding improving arrangement 50A. Thereby, the efficiency or light efficiency of the arrangement shown in FIG. 29B is improved when compared to the arrangement of FIG. 29A.
FIGS. 30A and 30B also describe illumination arrangements according to the present invention without and with the provision of the inventive light collecting and/or guiding improving arrangement 50a, respectively.
In the arrangement of FIG. 30A an air gap or air gap structure G is situated between the light exit aperture 30E of the light source device 10 and the light incidence aperture 50I of the light integrating device 50 in the form of an pyramidal integration rod 50. Produced ray C of light enters the internal of the light integrating device 50 after being transmitted by said air gap G under angle conditions which fulfill the TIR conditions of the light integrating device 50. Therefore ray C remains collected in the internal of said light integration device 50. However ray R shown in FIG. 30A gets lost by being reflected at the interface between the casing 30c of the light source 10 and the air gap G according to total internal reflection. This TIR condition at the interface between the casing 30c and the air gap G strongly depends on the large difference between the refraction indices of the material of the casing 30c or of the light source device material and the air gap.
To overcome this difficulty in the embodiment of FIG. 30b the air gap G between the light exit aperture 30E of the light source device 10 and the light incidence aperture 50I of the light integrating device 50 is filled with a liquid, gel, and/or a glue or a refraction index matching means 50r as said light collecting and/or guiding improving arrangement 50A or as a part thereof. Thereby, the light efficiency of the embodiment of FIG. 30B is improved when compared to the light efficiency of the embodiment of FIG. 30A, as for instance ray R is coupled to the integration rod 50.
FIGS. 31A and 31B demonstrate by means of cross-sectional side views embodiments of the inventive illumination arrangements without and with the provision of the inventive light collecting and/or guiding improving arrangement.
In both cases the illumination arrangement comprises as a light integration device 50 a hollow pipe or hollow guide tube in pyramidal form. This light integration device 50 comprises a light incidence aperture 50i which is positioned in the neighborhood of a light exit aperture 30E of said light source device 10. In a similar way as compared to the embodiment of FIG. 30A a ray C may be collected by said guide tube, whereas the ray R suffers from total internal reflection at the interface between the light source devices' material and the air. Therefore a fraction of the light or primary illumination light L1 being emitted from said light source device 10 via its light exit aperture 30E gets lost.
To overcome this problem a plain light pipe section 50p as said light collecting and/or guiding improving arrangement 50A or as a part thereof is provided filling an end section of the hollow guide tube or hollow pipe as said integration device 50. Additionally, said plain pipe section 50p completely fills the end section of the hollow pipe as said light integration device 50 and terminates the same and its light incidence aperture 50I. Between the end surface or light incidence aperture 501 and said light exit aperture 50E again a refraction index matching means 50r is provided so as to overcome the TIR problems at the interface between the light exit aperture 30E of the light source device 10 and the air gap G. Thereby less primary illumination light L1 is lost and additionally the light efficiency of the arrangement shown in FIG. 31B is improved over the light efficiency of the embodiment shown in FIG. 31A.
Reference Symbols1Illumination arrangement10light source device20light collecting, integrating and redirecting device30solid state light source device30ccase/case material, housing/housing material30Elight exit aperture, light output aperture30Ilight incidence aperture, light entrance aperture30Olight exit aperture, light output aperture31solid state light source, LED31-1solid state light source, LED31-2solid state light source, LED31-3solid state light source, LED31-4solid state light source, LED32solid state light source33array of solid state light sources40light valve device, LCD panel40Elight exit aperture, light output aperture40Ilight incidence aperture, light entrance aperture40Olight exit aperture, light output aperture50light integrating device, integrator rod, light pipe50Alight coupling and/or guiding improving arrangement50Elight exit aperture, light output aperture50E′light exit aperture, light output aperture50Ilight incidence aperture, light entrance aperture50I′light incidence aperture, light entrance aperture50mreflecting means, reflecting mirror50Olight exit aperture, light output aperture50pplain light pipe section, plain pipe section50rrefraction index matching means, liquid, gel, glue50sside wall50-1light integrating device, integrator rod, light pipe50-2light integrating device, integrator rod, light pipe50-3light integrating device, integrator rod, light pipe50-4light integrating device, integrator rod, light pipe55light mixing device, beam splitter device, colour cube device60display optics70projection optics80display, display screen81intermediate optics, lens arrangement82intermediate optics, polarization beam splitter1′illumination arrangement2′light source array2′1first light source2′2second light source2′3third light source2′4fourth light source2′5fifth light source3′pyramidal light pipe4′target surface10′first dichroic filter11′second dichroic filter12′first colour beam13′second colour beam14′third colour beam15′combined colour beam16′transmissive/reflective film17′transmissive/reflective film20′first prism21′second prism22′third prism23′first reflective/transmissive film24′second reflective/transmissive film25′first prism26′second prism27′third prism28′first reflective/transmissive film29′second reflective/transmissive film31′first prism32′second prism33′third prism34′fourth prism35′first transmissive/reflective film36′second transmissive/reflective film40′ illumination arrangement41′first light source42′second light source43′third light source44′colour cube45′first transmissive/reflective film46′second transmissive/reflective film47′pyramidal light pipe48′target surface49′output surface50′input surface51′output surface52′input surface53′first light ray54′second light ray55′third light ray56′fourth light ray60′ illumination arrangement61′first light source62′second light source63′third light source64′first dichroic filter65′second dichroic filter66′pyramidal light pipe67′target surface68′first transmissive/reflective film69′second transmissive/reflective film70′input surface80′illumination arrangement81′first light source82′second light source83′third light source84′colour cube85′first pyramidal light pipe86′second pyramidal light pipe87′third pyramidal light pipe88′target surface89′first transmissive/reflective film90′second transmissive/reflective film100′illumination arrangement101′first light source102′second light source103′third light source104′colour cube105′first transmissive/reflective film106′second transmissive/reflective film107′pyramidal light pipe108′target surface109′pyramidal light pipe110′pyramidal light pipe111′pyramidal light pipe120′semiconductor121′semiconductor active surface122′epoxy layer123′TIR ray124′TIR ray126′light pipe130′interface140′prism surface141′first transmissive/reflective (dichroic) film142′uncoated margin145′glass plate146′prism147′prismGgap structureG′gap structureL1primary illumination lightL2secondary illumination lightn1refraction indexn2refraction indexn3refraction indexRL1redirected primary illumination light